It is understood in the art that the complete uniform rotation of magnetic storage media within a magnetic field, including electronic or mechanical means to reverse field polarity, alter field orientation or gradually reduce magnetic strength, is generally an ideal means for erasing magnetically encoded information and promoting media reuse. However, it is also understood that the complete exposure of magnetic storage media to magnetic fields oriented approximately 90 degrees apart is an effective means for achieving quality erasure. Because there are a variety of magnetic storage media formats and media erasure standards, and because many degaussing applications tend more towards erasure and disposal, where media is erased to prevent unauthorized access to information when media is discarded (e.g., disposal of obsolete equipment, media erasure during a security emergency, etc.), than towards erasure and reuse, there are numerous bulk degaussing systems available that balance application requirements, size, cost, performance and the degree of operator skill required for proper use.
Bulk degaussing systems include handheld, tabletop, drop-through, conveyor-based, and cavity degaussing devices. Handheld, tabletop and simple conveyor-based devices typically require a human operator to manually rotate and possibly flip magnetic storage media between multiple passes through a magnetic field to achieve complete exposure and quality erasure. These devices are not well suited for high security applications since their performance will vary depending upon the diligence and skill of the human operator. Alternately, there are more complex drop-through and conveyor-based devices that employ arrays of magnetic poles generating multiple magnetic fields and permit quality erasure in a single pass. These devices do not require manual manipulation of magnetic media, but instead require substantially longer or more numerous magnetic field generating elements and longer degaussing paths. For example, a known conveyor-based device employs two pairs of opposed magnetic poles oriented at 90 degrees with respect to each other and at 45 degrees with respect to a degaussing path (FIG. 1). This configuration exposes magnetic storage media to two magnetic fields oriented approximately 90 degrees apart, but requires magnet pole and degaussing path lengths that are approximately three times longer than the intermediate dimension of the largest format media to be erased with the device. The longer pole and path lengths significantly increase the size and cost of such devices, and the increased cost can be especially significant if high strength rare earth permanent magnets are used in place of high power electromagnets. Cavity degaussing devices typically employ such high power electromagnets, which permit electronic manipulation of the orientation and strength of the magnetic degaussing field(s), but also require a reliable source of household or industrial current and some form of passive or active cooling during degaussing operation.
There is a need for improved bulk degaussing systems that have reduced size and cost, that reduce or eliminate electrical service requirements, that expose magnetic storage media to magnetic fields having varying orientations, and that reduce or minimize the degree of operator skill required to achieve quality erasure. U.S. Pat. Nos. 4,157,581 and 5,132,860 disclose bulk degaussing systems that rotate permanent magnets with respect to magnetic storage media while such media are translated through a magnetic field. Because magnetic domain switching can occur in a microsecond, a device may employ a rotating magnetic field to expose magnetic media to a variety of magnetic field orientations, thereby reducing the number of magnetic field generating elements required to produce quality erasure. However, such rotating magnetic fields can induce a sympathetic rotation in some magnetic storage media formats, which should be prevented to ensure quality erasure. For example, the '581 patent discloses a degaussing apparatus that employs projections that lock the hubs of magnetic tape cassettes to prevent rotation of tape or tape reels during an erasing operation. Such hub locking means tend to be format specific, which reduces the usefulness of the device or requires the manufacture and purchase of a variety of format specific media holders. Furthermore, such hub locking means cannot be used when a hub is inaccessible, as, for example, in most hard disk drives, where an internal spindle does not mechanically communicate with any external feature. Interactions between the rotating magnetic field and the spindle rotor in a hard disk drive can lock the platters in a single orientation with respect to a rotating magnetic field, which will prevent the recording surfaces from experiencing exposure to varying magnetic direction, and prevent quality erasure of the device.